DE 10 2010 050 428 A1 discloses a compression stage—heat storage power plant for the temporary storage of energy in the form of pressure energy in a compressible medium in a pressure accumulator and in the form of heat energy, with at least one injection pressure stage, which comprises at least one compression device and at least one heat exchange device connected in series, and at least one withdrawal pressure stage, which comprises at least one heat exchange device and at least one expansion device connected in series, wherein the number of injection pressure stages is not equal to the number of withdrawal pressure stages.
In many technical applications process heat is produced, which has to be dissipated. Examples of the utilisation of process heat are cogeneration plants and district heating. Often it would also be energetically advantageous to recover the heat. A direct heat recovery can be achieved for example with heat exchangers in the co-current and counter-current method, which often then enables high energy savings to be made.
With mechanical energy accumulators the heat recovery is an indispensable precondition for acceptable overall efficiencies. A dynamic method such as the co-current or counter-current method is however in this case not necessarily expedient, since the charging and discharging of the energy accumulator should as far as possible be chronologically decoupled from one another. Heat exchangers available on the market are characterised on the other hand by their continuous, simultaneous parallel or anti-parallel flow. The flow of the storage medium requires additional energy expenditure for the circulation, which can be saved.
If energy is stored in the form of compressed air, heat is necessarily produced in the compression (first law of thermodynamics).
It is simplest to calculate the heat for the example of an ideal isothermal compressor. This is theoretically around ⅓ of the expended compression work. In practice the heat is mostly released to the surroundings and the heat losses are in most cases also even considerably higher, for example ⅔ of the expended compression work. The invention is therefore aimed in particular at an adiabatic compressed air energy storage plant, (ACAES plant). In this connection the adjective adiabatic should not be understood here strictly in the thermodynamic sense (no heat exchange at all to the surroundings), but rather should simply mean that the heat generated in the compression should be recovered as far as possible.
Based on the thermal equation of state of ideal gases pV=nRT (the more realistic van-der-Waals equation simply introduces a correction of around 10%) and based on the caloric equation of state for a diatomic gas U=5/2nRT (air is 99% a diatomic gas) it is apparent that for a working gas in a predetermined volume V an increase in the pressure p is produced by raising the temperature T or the amount of substance n or both. In each case this is then equivalent to an increase of the internal energy U or enthalpy H=U+pV of the gas. R denotes the universal gas constant.
For an increase in energy or an increase in enthalpy of the system by supplying heat, it is effectively unimportant whether such a rise in pressure is produced by increasing the number of molecules in the pressure vessel or by raising the temperature, i.e. increasing the motion of the particles.
By adding and recovering the heat of compression overall efficiencies are possible in such a quasi-adiabatic system, which are theoretically far above those of conventional compressors and turbines.
In practice it is probably not possible to accomplish the adiabatic insulation of a giant high-pressure vessel. The fact is, if the thermal insulation were installed outside the pressure vessel, then the container wall would heat up. On account of the enormous heat capacity of the pressure vessel the heat of compression would then simply result in a slight rise in temperature of the container wall and at this low temperature would largely be lost for a practicable, direct heat recovery. Thermal insulation installed within the high-pressure vessel would on the other hand be subjected to enormous pressure and temperature fluctuations (several hundred bars, several hundred degrees Kelvin), which would destroy the insulation over time. Consequently the high-pressure vessel should be at the temperature of its surroundings.
This requires a necessary intermediate storage of the heat of compression. Another fundamental problem now arises: although heat can be efficiently transferred at high temperature differences and such temperature differences are in principle also advantageous in heat engines (see Carnot efficiency), the losses due to convection, conduction and in particular radiation however also increase correspondingly (proportional to the fourth power of the temperature, according to the Stefan-Boltzmann law).
High-temperature heat accumulator are in therefore principle unsuitable for longer periods of time, which however are basically desirable in energy storage. A known high-temperature accumulator suffers inter alia also from the fact that the discussed high temperature and pressure differences mechanically degrade the storage material (e.g. stone) and the resulting particles can on expansion sandblast the turbine blades.
Phase change materials (PCM) are not suitable for the temperature differences occurring in practice or are still in the experimental stage. They degrade over time and would therefore not be sufficiently efficient according to the present state of scientific knowledge. Also, the known industrial adsorption storage materials silica gel and zeolite are not suitable, since a large portion of the heat of compression is below the discharge temperatures and the heat transfer is a continuous and not a discrete process. With these materials only a relatively small amount of heat could therefore be recovered, which is above their discharge temperatures. The overwhelming amount of heat would be lost at a lower temperature.